As an alternative to traditional surgical vascular treatments where tissues are cut to reach a damaged artery or vein, "endovascular" treatments are now frequently used. Endovascular treatments are carried out at the lumen of the vessel. Some exemplary purposes and means for such treatments may be:
a) to produce artery or vein dilatation, PA1 b) to dissolve thrombus in their interior, PA1 c) to close abnormal communications of vessels among each other or with neighboring tissues, PA1 d) to carpet the surface of a vessel with a prosthesis, as a "sheathing", PA1 e) to return a dilated artery (aneurysm) to its normal caliber, or PA1 f) to isolate the inner surface of an artery from the blood chemical or physical elements, such as, for example, after performing dilatation with a balloon (internal bypass).
Endovascular expanders, commonly known as "stents", are often used to carry out the above techniques. Stents are tubular, permeable, elastic structures that are typically structured in special metallic meshes forming skeletal expandable tubes able to generate radial forces to keep vessels open.
Essentially, there are three general types of expanders or endovascular stents: thermosensitive stents, which adopt predetermined shapes at different temperatures, particularly that of the human body (such as, for example, as described in U.S. Pat. No. 4,425,908); stents expandable with a balloon (such as, for example, as described in European Patent EP 378.151), and stents that are self-expandable through structural elasticity (such as, for example, as described in U.S. Pat. No. 4,580,568 to Cesare Gianturco).
Self-expandable stents are typically compressed inside introductory devices or sheaths. Once the vascular area is reached, the sheath is removed, leaving the expander located endoluminally. Commonly used stents of this type are described in patents to Cesare Gianturco or assigned to Schneider (USA) Inc. of Plymouth, Minn. or Schneider (Europe) A.G. of Bulach, Switzerland.
Because of their structure, self-expanding stents typically experience longitudinal lengthening when compressed inside the sheath. When liberated inside the vascular lumen, they radially expand and longitudinally reduce. This change in shape poses a serious problem when the stent is combined with a Dacron or a polytetraethylene expandable prosthesis that covers the outside of the stent.
When the stent is deployed and expansion occurs, the length of the prosthesis remains essentially unchanged, while the stent noticeably shortens. For example, referring now to FIGS. 1 and 2, there are shown a compressed stent 12 and compressed prosthesis 13 in FIG. 1 and an expanded stent 12' and expanded prosthesis 13' in FIG. 2. FIGS. 1 and 2 illustrate the relatively substantial change in length of stent 12 upon expansion as compared to the insubstantial change in length of prosthesis 13.
Thus, a prosthesis that is the same length as the stent inside the sheath is too long when the stent is liberated. On the contrary, if the prosthesis is too much shorter than the stent inside the sheath, parts of the stent remain without prosthetic cover when the stent opens. Because of the different expansion properties between the prosthesis and the stent, and the frictional relationship between the two in the sheath, irregular expansion of the prosthesis may occur, provoking folds on the prosthesis that act as constrictor rings to limit the expansion of the expander.
Thus there is a need in the art for a deployment device that eliminates such problems associated with concurrent deployment of a stent and prosthesis.